egraphs: a few miscellaneous compile-time optimizations. (#5072)

* egraphs: a few miscellaneous compile-time optimizations.

These optimizations together are worth about a 2% compile-time
reduction, as measured on one core with spidermonkey.wasm as an input,
using `hyperfine` on `wasmtime compile`.

The changes included are:
- Some better pre-allocation (blockparams and side-effects concatenated
  list vecs);
- Avoiding the indirection of storing list-of-types for every Pure and
  Inst node, when almost all nodes produce only a single result;
  instead, store arity and single type if it exists, and allow result
  projection nodes to fill in types otherwise;
- Pack the `MemoryState` enum into one `u32` (this together with the
  above removal of the type slice allows `Node` to
  shrink from 48 bytes to 32 bytes);
- always-inline an accessor (`entry` on `CtxHash`) that wasn't
  (`always(inline)` appears to be load-bearing, rather than just
  `inline`);
- Split the update-analysis path into two hotpaths, one for the union
  case and one for the new-node case (and the former can avoid
  recomputing for the contained node when replacing a node with
  node-and-child eclass entry).

* Review feedback.

* Fix test build.

* Fix to lowering when unused output with invalid type is present.

cranelift/codegen/src/machinst/lower.rs

@@ -381,7 +381,7 @@ impl<'func, I: VCodeInst> Lower<'func, I> {
             for inst in f.layout.block_insts(bb) {
                 for &result in f.dfg.inst_results(inst) {
                     let ty = f.dfg.value_type(result);
-                    if value_regs[result].is_invalid() {
+                    if value_regs[result].is_invalid() && !ty.is_invalid() {
                         let regs = alloc_vregs(ty, &mut next_vreg, &mut vcode)?;
                         value_regs[result] = regs;
                         trace!(